1. Field of the Invention
This invention relates to a method of and apparatus for decoding a currently received one of a series of two-layer coded video signals each of which includes a base layer of coded base data and coded motion vector data, and an enhancement layer of coded enhancement data being the differences between the uncoded video signal and a reconstruction of the immediately previous base video signal from the base layer.
2. Related Art
In this application the phrase video signals refers to a digital representation of a video source.
This so called two-layer coding of video signals was devised for use on ATM networks which may suffer cell loss causing large disruption in the transported data streams on variable bit rate (VBR) channels.
An example of a two-layer coding scheme is disclosed in U.S. Pat. No. 4,933,762 in which transmission is via a main path and an auxiliary path of a variable rate digital transmission network. When the main path is not capable of dispatching the totality of the picture information, auxiliary information is dispatched via the auxiliary path.
A drawback of conditional replenishment systems where only the significant differences between successive video signals (or frames) are coded and transmitted when more efficient interframe coders are used is that transmission errors are not confined to a single decoded frame but propagate into later frames.
Masahiro Wada in an article titled "Selective Recovery of Video Packet Loss Using Error Concealment", IEEE Journal on Selected Areas in Communications, Vol 7 (1989) Junek no 5, New York US describes an error concealment method for a single channel (i.e. one layer) transmission scheme over a VBR channel. When a decoder detects a video packet loss, picture quality is partially recovered by motion-compensated error concealment in which blocks from the previously received video frame are shifted by the average motion vectors of the blocks which neighbour those used for concealment. This method may prove adequate for one or two missing blocks but may cause a greater disturbance for a larger group of missing blocks. Experiments by the Applicant have shown that as many as four blocks can be fitted into a cell, or packet, and when cell loss occurs on an ATM network, it usually happens in bursts of cells, perhaps between three and ten, resulting in tens of missing blocks.
Preferably, the base layer carries data for a base mode picture together with vital information such as synchronisation data and should, therefore, be transmitted at a constant bit rate on a "guaranteed" constant bit rate channel.
The enhancement layer coding the differences between the video signal being coded and a video frame reconstructed from the base layer alone (which will also be referred to as "enhancement data"), brings the base mode picture up to the final required quality. The enhancement layer will, in general, have a fluctuating bit rate and can be transmitted via a VBR channel. Should any of this information be lost it will not affect tracking of the reconstructed frames but will only result in a momentary reversion to base mode quality. The occasional loss of resolution is compensated by obtaining the benefits of VBR transmission, for example, cost efficient use of the network channels.
Such coded video signals can be decoded by decoding the base layer to obtain a base video signal, decoding the enhancement layer to obtain enhancement data, and combining the base video signal and the enhancement data to obtain the final decoded video signal.
This two-layer coding scheme has been verified by computer simulation on a two-layer adaption of the CCITT H. 261 standard as reported in the IEEE Journal on Selected Areas in Communications, Vol 7 No. 5 June 1989 in an article by M. Ghanbari titled "Two-layer Coding of Video Signals for VBR Networks" in which the inclusion of motion compensation to code the video signal in the base layer leads to a high degree of bandwidth compression. The motion vectors are transmitted as part of the base layer in a "guaranteed" constant bit rate (CBR) channel as they are essential for reconstructing the base mode picture. As the enhancement layer mostly carries information about picture details or edges, the loss of data from this layer is seen as loss of resolution in the reconstructed picture generated from the decoded video signal. The base layer data will deliver an acceptable picture quality for plain areas but more detailed areas may suffer visible degradation, although computer simulations indicate this particular H. 261 based scheme is resilient to cell loss with rates as high as 1 in 10 not being readily visible.